The problem I would see it having is in cycling.
Blackpowder is lower in power (per volume that is-it takes a LOT of it).
There would not be enough cycle power for the gas piston or spring.
Not from such a tiny cartridge loaded with black.
The spring and stuff would have to be equal to the ak-22.
Second-that thing would foul up and quit cycling within just a few shots. No kidding.
The gatling gun of that era, was only able to work, because of the many barrels and a non-self loading mechanism---in other words--you had to crank it.
Nice idea though!!!! I love thinking about how we could have won that war.